1. Field of the Invention:
This invention relates to hydraulic fracturing of earth formations, and more particularly to the hydraulic fracturing of HC (hydrocarbon) bearing formations, e.g. oil and gas sands, for the purpose of increasing the producing rate and total amount of recovery of the hydrocarbons from a well completed in such a formation, and, in the case of storage wells, for the purpose of increasing the injection rate and total capacity.
2. Brief Description of the Prior Art:
The hydraulic fracturing of HC formations is well known and is described for example in the following United States patents number:
3263751 - Kiel et al. PA1 3376930 - Kiel et al. PA1 3373815 - Kiel et al. PA1 3378074 - Kiel PA1 339727 - Graham & Kiel PA1 3444889 - Kiel et al. PA1 3497008 - Graham & Kiel PA1 3553494 - Kiel PA1 3601198 - Ahearn & Kiel PA1 3664420 - Graham, Kiel & Terry PA1 3695355 - Wood & Kiel PA1 3700032 - Terry, Graham, Sinclair & Kiel PA1 3722595 Kiel
And in the references cited against the above listed patents. Further description of the subject is to be found in an article entitled "Reservoirs in Fractured Rock" by Stearns and Friedman appearing at pages 82 et seq. of AAPG Memoir No. 16 Stratigraphic Oil & Gas Classification Methods and Case Histories - 1972 and the bibliography appended thereto.
The function of fracturing is to overcome the deficiency in permeability of the formation adjacent the well bore by creating a highly conductive path reaching out into the reservoir rock surrounding the well bore. According to the usual practice, a fluid such as water, oil, oil/water emulsion, gelled water, or gelled oil, is pumped down a well bore with sufficient pressure to open a fracture in the HC formation. The fluid may carry a suitable propping agent, such as sand, into the fracture for the purpose of holding the fracture open after the fracturing fluid has been recovered, e.g. allowing the well to flow. A normal fracture treatment consists of one continuous injection of fluid. In the case of tight, i.e. low permeability wells, i.e. below 1 md permeability, fracturing produces results that are of but a temporary nature as far as increasing rate of flow is concerned and little or no increase in total recovery is achieved. After perhaps a short period of accelerated flow, rate of production may drop off to near previous levels. Repeated stimulation with the same or similar procedure may again produce but a temporary gain.
Prior to the present invention, a conductive fracture extending radially one hundred fifty feet from the well bore was believed to be about the maximum obtainable.
The reason for the temporary increase in productivity produced by the prior art is believed to be that the fracture communicates the well bore with a small portion of the joint system between the matrix elements of the formation and with a small portion of the reservoir matrix. However, as soon as this low volume space has been drained, productivity drops off to that controlled by the low permeability reservoir matrix, and since the area exposed to such matrix by a short fracture is low, productivity is low.
Hydraulic fracturing procedure usually has best results in formations of moderate permeability, e.g. one to twenty millidarcies. In order to achieve satisfactory production from a formation of low permeability, e.g. below one millidarcy, it has been the belief of experts that a much longer fracture than that heretofore attainable is necessary.